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(EMI) in Power Supplies Alfred Hesener Electromagnetic Interference (EMI) in Power Supplies Alfred Hesener Abstract -- Increasing power density, faster switching and higher been widely accepted. The sensitivity to load and line currents forces designers to spend more time both considering the effects of electromagnetic interference (EMI) and debugging a design regulations can limit their usage and parameter that has EMI problems but is otherwise complete. This paper explains variations of passive components can make series the different types of EMI and their coupling mechanisms and the existing EMI regulations. The most frequent noise sources, production difficult and expensive. Further, for some transmission paths and receiver sensitivity are examined. Based on stages of the power supply (e.g. secondary side post- real designs and measurements, specific procedures are recommended for use throughout the design cycle, to make the power regulation) a resonant version does not really exist. It is supply work reliably and pass EMI testing. only with today’s modern control ICs that quasi-resonant power supplies show their potential while maintaining I. INTRODUCTION good EMI performance. So it is not surprising that more In power supplies, the two prominent types of EMI and more designs are using this topology. are conducted EMI and radiated EMI. Comprehensive Given these new developments, it is clear that EMI regulations provide limitations to radiated and conducted performance can no longer be considered only after the EMI generated when the power supply is connected to main power supply design is finished. It needs to be the mains. designed into the power supply right from the start at Comparing the modern power switches used in power specification level, just like reliability and safety, supplies with those from older generations, the new influencing topology and component selection. switches have significantly reduced switching times, The goal is to meet EMI regulations while not leading to faster and faster rise and fall times for the disturbing other applications nearby. The power voltage and current waveforms. These fast edges supply should also be self-compliant and tolerate a produce significant energy at surprisingly high certain amount of EMI from the outside. This paper frequencies, and are the root cause of all EMI problems will show how to “embed” EMI considerations in switched-mode power supplies. This high frequency throughout the entire design cycle. The intent is to energy causes ringing in all the resonant tanks, small or give the power supply designer a good large, that exist within the power supply. In general, this understanding of the problem and an overview of wringing does not cause problems; however, in some the measures that can be taken while designing and cases, this may stop the power supply from working testing the power supply, to improve time to market properly or passing tests. and to come up with a robust design. It is not a Faster switching also means that losses can be reduced, comprehensive overview on the topic, as a large improving the efficiency of the power supply. But faster amount of good literature exists already.[1]-[4] switching should also enable higher switching frequencies, ultimately leading to smaller passive II. DIFFERENT TYPES OF EMI AND components and better transient behavior – a promise THEIR CHARACTERISTICS that has not been realized. Three things can cause an EMI problem: A signal The main reasons for this are the cost of transformers source creates some kind of noise, there is a transmission for use at these frequencies and the disproportional path for the noise, and/or there is a receiver sensitive complexity of solving high frequency EMI problems. enough to be distorted by the noise, as shown in Figure 1. Resonant and quasi-resonant topologies offer an elegant way out of this dilemma. They have been around for a long time, but due to limitations, they have not Fairchild Semiconductor Power Seminar 2010-2011 1 Shielding the source using thin conductive layers is most effective. Inductive coupling: This transmission path is quite common in switched-mode power supplies since high- frequency currents in the inductors can cause strong magnetic fields at higher frequencies, where the Fig. 1. EMI sources. coupling factors can be higher. Magnetic shielding is The noise source can be inside or outside the power less effective than electric shielding since the absorption supply. Tackling the noise problem at the source means depth is smaller, requiring thicker materials. Inductive reducing the emission levels — for example, by coupling is best addressed at the source. lowering noise amplitudes. Different coupling Wave coupling: Here, the noise typically has a high mechanisms exist for noise, and many EMI frequency, and is transmitted via an electromagnetic countermeasures focus on these; however, they overlook wave. It does not play a major role in power supplies, what can be done at the emitter or receiver. A receiver since frequencies are not high enough, and can be susceptible to noise injection must exist in the system if damped very effectively with shielding. there is an EMI problem. Here, the obvious solution is This paper will focus on capacitive, resistive, and reducing its sensitivity. inductive coupling; as they are the most important At this point, a fundamental distinction must be made sources of EMI issues in power electronics applications. between the two types of EMI problems: It is generally accepted industry practice to consider - Improving EMI so that the design meets conducted EMI below 30MHz, radiated EMI above regulations and will pass EMI testing (also 30MHz, and in most cases up to 1GHz – exceptions do called EMC or electromagnetic compliance) exist, however. - Improving EMI so that the design works reliably Coupling modes cannot be treated in isolation since in all modes of operation, with good efficiency, ideal elements exist only in simulators, not in real life. and does so without being disturbed by other Parasitic elements are always present. The parasitic (EMC-compliant) equipment nearby capacitors and inductors contribute to the problem, as For the first type, test methods and certified labs exist. parts of tank circuits that will resonate when stimulated For the second type, it is important to take the design by a voltage or current edge. The parasitic tanks help to through all design stages, carefully checking to see if convert one coupling mode into another, and that is why poor EMI design may be the cause of the problem. Here, coupling modes cannot be analyzed and fixed in it is important to consider component variations. Maybe isolation. The third parasitic element, resistance, actually the components in the prototype are such that no helps to ease the problem by damping the resonant problem is visible, but the components used in oscillation. Using the amplitude change from peak to production may cause problems. peak can help to calculate the parasitic resistance, The four coupling mechanisms are: identify it in the circuit, and optimize the circuit Resistive (or galvanic) coupling: The noise signal is accordingly. transferred via electrical connections. This works at all frequencies, and is usually fixed by good layout III. REGULATIONS AND STANDARDS FOR (particularly the ground layout) and filtering with EMI capacitors and inductors or lower signal levels with RC As electrical consumers moved from simple light elements. “Common impedance” coupling can be bulbs to large motors and particularly to switched-mode classified as galvanic coupling. power supplies with rectifiers and capacitors at the Capacitive coupling: Electrical fields are the main inputs, the quality of the grid voltage and service transmission path. Capacitance levels are mostly small worsened. This led to the emergence of worldwide so this affects small signals and/or high frequencies. Fairchild Semiconductor Power Seminar 2010-2011 2 standards to mitigate these problems. Two measured and appropriate limits defined. Using a considerations of these standards are: defined impedance to measure the harmonic current is - Limit the amount of emission not required today, but is also under discussion. (radiated/conducted) which a given application The standard 61k-3-2 defines the limits for generates applications <16A, and 61k-3-12 for 16A…75A current - Define the minimum immunity levels consumption. The standard 61k-4-7 defines the (radiated/conducted) a given application must measurement and evaluation method. Additional 61k tolerate without malfunction standards define the behavior for voltage and frequency The list of standards is very long. The common theme variations, immunity to conducted and radiated radio is that certain standards define the limit values and their frequency signals, fast voltage transients, surges, voltage measurement methods and conventions, and additional dips / drops, short interruptions and flicker, and documents define the regulations for classes of magnetic fields. applications in more detail. For the standard 61k, the equipment typically is Additionally, standards can be grouped into grouped into different classes, with professional local/regional standards, MIL standards, automotive equipment above 1000W power consumption still being standards, standards for the aircraft industry, for excluded. Different limit levels exist for the different physically large equipment, and for more specialized classes. An overview is given below. equipment (e.g. smart meters). In most cases, a good
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